This invention relates to an emissions unit for sampling exhaust gases, and more particularly, the invention relates to a particulate sampler used in analyzing the particulate matter in exhaust gases.
Particulate samplers have been used to collect particulate material found in exhaust gases and convey them to a filter or analyzer. A full flow method measures the particulates by diluting all of the exhaust gases expelled from the vehicle. However, this particulate sampling method is very costly. Alternatively, a partial flow particulate sampling method has been used in which a small probe is inserted into the tailpipe to obtain a small sample of the exhaust gases. This smaller fraction of exhaust gas is then diluted to obtain a temperature of the mixture below 125° F. (52° C.). The particulate matter in the diluted sample is measured and calculations are made to determine the total amount of particulate matter present in all of the exhaust gases expended from the vehicle. The partial flow method is much more cost effective, however, it is subject to variations due to an inability to accurately account for all of the particulate matter in the sample exhaust.
The structure of the passageways within the particulate sampler has an impact upon the accuracy of the particulate matter measurement. For example, a fraction of the particulate matter can collect on the walls of the passageways and, therefore, never reach the filter or analyzer. As a result, the particulate matter measured will be lower than the actual particulate matter in the sample exhaust gases.
The sampled exhaust gases typically are pulled through a probe to a mixer where dilution gases are introduced to the exhaust gases. From the mixer, the exhaust gases and dilution gases travel through a passageway where they are homogeneously mixed together. It is desirable to maintain the exhaust gases as close to its original exhaust gas temperature as possible to ensure accuracy. During a typical emissions test, the temperature of the exhaust gases fluctuates. Accordingly, it is desirable to accommodate these temperature fluctuations in exhaust gases during the test. That is, it is desirable that the particulate sampler collects the sample without changing the temperature of the exhaust gases until it reaches the mixing passageway. Therefore, it is desirable to provide a particulate sampler that maintains the temperature of the exhaust gases during the test while conveying all the particulate matter to the filter or analyzer.